Multi-radiator antennas are frequently used in for example cellular networks. Such multi-radiator antennas comprise a number of radiating antenna elements for example in the form of dipoles for sending or receiving signals, an antenna feeding network and an electrically conductive reflector. The antenna feeding network distributes the signal from a common coaxial connector to the radiators when the antenna is transmitting and combines the signals from the radiators and feeds them to the coaxial connector when receiving. A possible implementation of such a feeding network is shown in FIG. 1.
In such a network, if the splitters/combiners consist of one junction between e.g. 3 different 50 ohm lines, impedance match would not be maintained, and the impedance seen from each port would be 25 ohm instead of 50 ohm. Therefore the splitter/combiner usually also includes an impedance transformation circuit which maintains 50 ohm impedance at all ports.
A person skilled in the art would recognize that the feeding is fully reciprocal in the sense that transmission and reception can be treated in the same way, and to simply the description of this invention only the transmission case is described below.
The antenna feeding network may comprise a plurality of coaxial lines being substantially air filled, each coaxial line comprising a central inner conductor at least partly surrounded by an outer conductor with insulating air in between. The coaxial lines and the reflector may be formed integrally with each other in the sense that the outer conductors and the reflector are formed in one piece.
Antenna feeding networks of the closed type are known, i.e. feeding networks where the outer conductor in each coaxial line forms a cavity around the central inner conductor, i.e. encircles or forms a closed loop around the central inner conductor as seen in a cross section perpendicular to the longitudinal direction of the coaxial line, see FIG. 2. One disadvantage with such a closed antenna feeding network is that it may be difficult to assemble the antenna, e.g. properly arranging the central inner conductors and associated components such as support means for holding the inner conductors and connection means between the inner conductors inside the outer conductors. Furthermore, if movable dielectric elements are provided between the outer and inner conductors to provide a phase shifting functionality, the positions of such dielectric elements are not easily adjustable due to the closed outer conductors.
Antenna feeding networks of the open type are also known, i.e. feeding networks where the outer conductors in at least some coaxial lines are provided with openings, and thus do not completely surround or encircle the inner conductors. One example of such a feeding network is disclosed in WO2005/101566 in which an antenna feeding network having coaxial lines with a longitudinally extending opening along one side of the outer conductor, see FIG. 3. The inner conductors are supported by dielectric support means. Pairs of adjacent inner conductors may be interconnected by cross-over elements are arranged in openings through the wall between the inner conductors. This feeding network solves some of the problems associated with the closed type feeding network, in particular it is easier to assemble since direct access to the interior of the coaxial lines is provided. On the other hand, the longitudinally extending openings makes the antenna less mechanically stable and unwanted backwardly directed radiation may occur. Such unwanted radiation may reduce the antenna performance in terms of e.g. back- or sidelobe suppression. In antennas having two cross-polarized channels, it may also reduce cross-polarisation isolation and also isolation between the two channels. All those antenna parameters may be important to the performance of e.g. a cellular network in terms of e.g. interference and fading reduction. The problem with unwanted radiation may be solved at least in part by additional components in the form of conductive covers to cover the cross-over elements. Using such covers add to the cost and complexity of the feeding network however.